Gas turbine engines operate by passing a volume of gases through a series of compressors and turbines in order to produce rotational shaft power. High energy gases rotate a turbine to generate the shaft power. The shaft power drives a compressor to provide compressed air to a combustion process that generates the high energy gases for turning the turbine. In an aircraft engine, the turbine can be used to generate propulsion, such as directly via thrust, or indirectly via a shaft and fan or propeller. In an industrial gas turbine, the shaft power can drive a generator that produces electricity. Alternatively, a power turbine can be used to drive a shaft for powering the generator.
Each compressor and turbine includes a plurality of stages of stators and blades, each stator and blade including an airfoil. In general, stators redirect the trajectory of the gas coming off the blades for flow into the next stage. In the compressor, stators convert kinetic energy of moving gas into pressure, while, in the turbine, stators accelerate pressurized gas to extract kinetic energy. In the compressor, the rotating blades push gas past the stationary stators. In the turbine, the rotating blades extract rotational power from the flowing gas.
Typically, airfoils used in turbines and compressors, particularly turbines, are subject to extremely high temperatures, often times exceeding the melting point of the base alloy of the blade. As such, turbine blades are typically supplied with a variety of coatings, such as metallic bond coatings and thermal barrier coatings that protect the base alloy from the high temperatures. Turbine components may need to meet high-precision tolerances. For example, a blade may need to fit the particular dimensions of the engine to which it is mounted. Accordingly, the blade tips may be ground after coating and prior to installation to ensure best clearances and optimal performance. Periodically, these coatings need to be reapplied to turbine blades at regular maintenance intervals or in the event of damage and may require post-coating grinding to meet the appropriate tolerances.
Various attempts have been made to provide thermal barrier coatings with surface features for various purposes, as are described in U.S. Pat. No. 6,703,137 to Subramanian, U.S. Pat. No. 7,686,570 to Allen, and U.S. Pat. No. 8,021,742 to Anoshkina et al.